1. Field of the Invention
The present invention relates to a wiring structure of an integrated circuit that is formed on a substrate having an insulating surface by using thin film transistors (hereinafter called TFTs). The invention also relates to a wiring structure of a liquid crystal display device of a peripheral circuits integration type that is formed on a substrate having an insulating surface by using TFTs.
2. Description of the Related Art
A technique is known in which a silicon film having crystallinity is formed on a glass substrate or a quartz substrate and a TFT is formed by using the silicon film thus formed. This type of TFT is called "high-temperature polysilicon TFT" or "low-temperature polysilicon TFT."
The high-temperature polysilicon TFT is formed on a highly heat-resistant substrate such as a quartz substrate because in order to obtain a crystalline silicon film with an active layer heating at 800.degree. C. to 900.degree. C. is required. On the other hand, the low-temperature polysilicon TFT is formed on a substrate that is relatively low in heat resistance such as a glass substrate by a process requiring less than 600.degree. C.
A high-temperature polysilicon TFT has the advantage that TFTs similar in characteristics can easily be integrated on a substrate, and that it can be manufactured by utilizing the various process conditions and manufacturing apparatuses of conventional IC processes. On the other hand, a the low-temperature polysilicon TFT has the advantage that a glass substrate may be used that is inexpensive and can easily be increased in size (large-area substrate).
According to current technologies, there are no large differences in characteristics between a high temperature polysilicon TFT and the low-temperature polysilicon TFT. Both types of TFTs provide mobility values of about 50 to 100 cm.sup.2 /Vs and S-values of about 200 to 400 mV/dec (V.sub.D =1 V).
Techniques of producing a liquid crystal display device where integrated circuits, an active matrix circuit, and peripheral circuits for driving the active matrix circuit, are formed on the same substrate (also known as a peripheral circuits integration type liquid crystal display device") are now being studied.
However, the characteristics of the conventional high-temperature polysilicon and low-temperature polysilicon TFTs are much poorer than those of a MOS transistor formed on a single crystal silicon wafer. Typically, a MOS transistor form on a single crystal silicon wafer yields an S-value of 60 to 70 mV/dec.
Furthermore, in both high-temperature polysilicon and low-temperature polysilicon TFTs according to the current technologies, because of low mobility, the driving frequency of such TFTs itself is obliged to be less than several megahertz.
For example, where peripheral circuits of a liquid crystal display device are formed by using high-temperature or low-temperature polysilicon TFTs, it is impossible to directly input, (to drive the TFTs), a clock signal or a video signal of more than tens of megahertz necessary for display.
For the above reason, a plurality of wiring lines (Interconnections) are used to transmit clock signals or video signals and the clock signals or video signals are supplied to the TFTs in such a manner as to be reduced in frequency (called divisional driving). For example, a 10-MHz frequency of an original clock signal is divided into 2.5 MHz by using four wiring lines. The respective TFTs are driven at this low frequency. This increases the number of wiring lines and the number of TFTs, resulting in problems such as increased installation area.
The present inventors have developed a TFT which exhibits performance equivalent to that of the MOS transistor using a single crystal silicon wafer though it uses a crystalline silicon film.
Such a TFT uses a crystalline silicon film having a crystal structure that is continuous in a predetermined direction, for instance, in the source-drain direction as well as having grain boundaries extending in the same, predetermined direction.
This type of crystalline silicon film is obtained by introducing a very small amount of a metal element (for instance, nickel) for accelerating crystallization into an amorphous silicon film, then heating the amorphous silicon film at 500.degree. to 630.degree. C. (for instance, 600.degree. C.) to cause lateral crystal growth, and thereafter forming a thermal oxidation film.
Having much superior characteristics such as an S-value of smaller than 100 mV/dec and mobility of higher than 200 cm.sup.2 /Vs, this type of TFT, in itself, can be driven at tens to hundreds of megahertz or even higher frequencies. By using this type of TFT, TFTs capable of being driven at high speed can be integrated on a large-area substrate.
As a result, not only can circuits having much superior performance be obtained but also the numbers of thin-film transistors and wiring lines necessary for driving can be reduced to a large extent from the conventional cases, thereby greatly contributing to miniaturization and increase in the degree of integration of devices.
However, where an integrated circuit is formed by using TFTs over such a large area as a several centimeter square to a tens of centimeter square as in the case of the peripheral circuits integration type active matrix liquid crystal display device, the rounding of high-frequency signals that are transmitted by wiring lines becomes a very serious problem when such integrated circuit is driven at a high frequency such as tens to hundreds of megahertz or higher.
This problem will be described below for peripheral circuits of a liquid crystal display device. FIG. 5 is a top view of a peripheral circuits integration type active matrix liquid crystal display device.
As shown in FIG. 5, an opposed substrate 902 having an opposed electrode (not shown) on its inside surface is opposed to a substrate 901 with liquid crystal material (not shown) interposed in between.
A data lines (source lines) driving peripheral circuit 903, a scanning lines (gate lines) driving peripheral circuit 904, and an active matrix display section 905 in which respective pixels are provided with pixel electrodes and switching TFTs that are connected to the respective pixel electrodes are provided on the substrate 901.
A flat cable 906, which extends from external circuits to supply signals to the liquid crystal display device, is electrically connected to peripheral wiring lines 907 at an end portion of the substrate 901. The peripheral wiring lines 907 are connected to wiring lines 908 and 909 in the peripheral circuits 903 and 904. The peripheral wiring lines 907 and the wiring lines 908 and 909 in the peripheral circuits 903 and 904 are arranged parallel or approximately parallel with each other.
The wiring lines 907 to 909 are formed as thin films of a conductive material such as aluminum, and at the same time as the TFTs of the peripheral circuits 903 and 904 and the active matrix circuit of the display section 905.
Part of the wiring lines 907 to 909 are used for transmitting a signal of a very high frequency, for instance, more than 10 MHz. Typical examples of those wiring lines are a video signal line for transmitting a video signal and a clock signal line for supplying a clock signal.
In general, the clock signal frequency amounts to about 12.5 MHz in the case of VGA (640.times.480.times.3 (three colors of RGB) pixels), and the video signal frequency increases with the clock signal frequency, such as when the image resolution becomes higher.
In particular, in the peripheral circuits integration type liquid crystal display device, the peripheral circuits 903 and 904 which drive the display section 905 which might be several centimeter square to a tens of centimeter square are usually provided alongside display section 905 and hence have similar length.
Each of the peripheral circuits 903 and 904 has wiring lines that extend from one end to the other within the circuit.
The clock signal line and the video signal line are examples of such wiring lines. Such wiring lines may have a length of several centimeters to tens of centimeters inside the peripheral circuits 903 and 904.
The electric resistance of long wiring lines becomes very high even if made of a material having high electric conductivity, such as aluminum.
The peripheral wiring lines 907 for transmitting signals from flat cable 906 to peripheral circuits 903 and 904 also have line width of tens to hundreds of micrometers and length several centimeters or more, even tens of centimeters in some cases.
In view of the length of the peripheral wiring lines 907 and the length of the wiring lines 908 and 909 in the peripheral circuits 903 and 904, it is understood that signals are transmitted by wiring lines of length not available in such scale in conventional IC chips.
On the other hand, capacitance coupling is likely to occur in parallel-arranged wiring lines when a high-frequency signal is applied thereto because they are distant from each other by only tens to hundreds of micrometers.
Further, in the liquid crystal display device, the opposed electrode (not shown) is provided on the entire surface of the opposed substrate 902. From the viewpoints of protecting the peripheral circuits 903 and 904 and simplifying the manufacturing process, it is common design to provide not only the display section 905 but also the peripheral circuits 903 and 904 and the peripheral wiring lines 907 on the surface that confronts the opposed substrate 902.
Therefore, the opposed electrode confronts the peripheral wiring lines 907 and the wiring lines 908 and 909 in the peripheral circuits 903 and 904, and hence capacitance coupling may occur between the opposed electrode and the above wiring lines.
The capacitances formed between wiring lines or between wiring lines and the opposed electrode (provided on the inside surface of the opposed substrate 902 that confronts the substrate 901 via the liquid crystal) and the high resistance of each wiring line cause deterioration, i.e., rounding, of a transmission signal waveform. That is, a signal that is transmitted by a wiring line, even when it has a good shape (for instance, a rectangular shape) at the input stage, is more rounded (the rising position of the waveform is delayed or the waveform is disordered) as it reaches the end of the wiring line.
When a signal waveform is rounded to a large extent, a delay may occur in the operation timing of a circuit or erroneous video information transmitted to pixels, possibly resulting in erroneous operation or a disordered image.
This problem becomes more serious as the size of the display section 905 increases or the driving frequency is increased such as by increasing the display resolution.
Among the peripheral circuits 903 and 904, the rounding has a great influence, i.e., is a serious problem in the circuit 903 when driving the data lines (source lines) because it is supplied with high-frequency signals of tens to hundreds of megahertz.
At present, integrated circuits in the form of a chip that use a single crystal silicon wafer are also common and operate at a driving frequency of tens to hundreds of megahertz. However, in such cases, since the entire integrated circuit is accommodated in a chip of an about 1-to-2-cm square, wiring lines are short and hence the rounding is less serious than in the large-area liquid crystal display device.
To reduce the capacitance between wiring lines, it is necessary to increase the distance between the wiring lines and decrease the dielectric constant of the region between the wiring lines.
However, if the distance between wiring lines is increased, the area necessary to accommodate the wiring lines and a circuit that uses the wiring lines increases, resulting in an increase in the size of the entire device. Thinning the width of wiring lines is not favorable either however, because the electric resistance increases as the distance between the wiring lines decreases.
The distance between the wiring lines and the opposed electrode is relatively small (the interlayer insulating film is 1 to 2 .mu.m thick and the liquid crystal layer is 3 to 8 .mu.m thick, and hence the total thickness is about 10 .mu.m). However, the thickness of the liquid crystal material layer, i.e., the cell gap, cannot be increased for optical reasons. It is difficult to increase the distance between the wiring lines and the opposed electrode sufficiently to obtain a desired reduction in capacitance by increasing the thickness of the interlayer insulating layer.
As described above, it is difficult for the current technologies to effectively reduce the capacitance between wiring lines.
One would think the electric resistance of wiring lines can be reduced by widening or thickening the wiring lines. However, thickening the wiring lines is unfavorable because it makes hillocks to occur more easily due to heating when manufacturing. As a result, short-circuiting occurs more easily between wiring lines that cross each other via the interlayer insulating film.